Bash Adding 09 or 9.1 etc

I have a script that gives a error in BASH whenever I try to add 01-09
with 1.

X="09"
X=$((X+1))

line 42: 09: value too great for base (error token is "09")

This is caused by a date command variable and I'm trying to avoid
having to make other changes in the script.


TIA





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